A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and scanners, in which each target portion is irradiated by scanning the pattern through the radiation beam in a given direction (the “scanning” direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
A mirror array typically employs a matrix arrangement of small reflective elements (or mirrors), which are individually adjustable, for example tiltable about an axis (by an actuator), so as to apply a pattern to a reflected beam of radiation. Mirror arrays may alternatively be referred to as a “programmable mirror array”, a “micromirror array” or an “active faceted mirror.”
It should be appreciated that such mirror arrays have several uses and, in particular, may be utilized in lithographic apparatus. For example, it is known to utilize mirror arrays to form the patterning device of a lithographic apparatus (for example in a lithographic apparatus for use in the manufacture of flat panel displays).
More recently, the use of mirror arrays in the illumination system of a lithographic apparatus has been proposed. The illumination system of a lithographic apparatus is arranged to receive radiation from a source (for example a laser) and produce an radiation beam for illuminating an object (for example a patterning device). The illumination system shapes and controls the radiation beam to provide a beam with a desired spatial intensity distribution and angular intensity.
A conventional illumination system may comprise a diffractive optical element (“DOE”) and a “zoom-axicon” apparatus (which is a device configured to adjust the intensity distribution at a pupil plane). Several disadvantages have been identified with such conventional illumination systems. For example, to produce the desired range of illumination settings the zoom-axicon module will generally have several (e.g. five or more) optical components, which can make it expensive to produce, particularly given the fact that several of the elements must be independently movable. A further problem is that the lenses of the axicon (which may for example comprise a zoom lens and two conical elements) represent a considerable thickness of lens material and a large number of surface interfaces such that transmission efficiency may be poor due to absorption, reflection, inefficient coatings, degradation effects and contamination. This problem is exacerbated by the demand for imaging ever smaller features at higher densities which requires the use of radiation with shorter wavelengths, such as 193, 157, 126 nm or even EUV (e.g. 5-20 nm). Thus, a mirror array based illumination system is desirable.
Mirror array based illumination systems are more flexible and are faster than the prior art combination of diffractive optical element and zoom-axicon. For example, changing an illumination mode generated using a prior art diffractive optical element requires several seconds, since the diffractive optical element must be replaced. The mirror array based illumination system allows the illumination mode to be changed more quickly. Furthermore, the prior art zoom-axicon can only make circularly symmetric changes to the spatial intensity, whereas the mirror array based illumination system does not have this limitation.
However, Applicants have recognized a number of problems associated with mirror arrays, particularly when used in lithography. Since the individual reflective elements of a typical mirror array are generally very small, for example a mirror array may comprise over a 1000 microscopic mirrors, the elements may be susceptible to damage during use. For example, heat generated by the radiation (that the mirrors are reflecting) may cause the reflective elements to be damaged by overheating. Such heat generation is, for example, particularly noticeable with the high optical power and short wavelengths used in deep UV and EUV applications. Furthermore, damage may be caused during movement of the reflective elements, for example by excessive oscillation of the reflective element following actuation.